Limit switch



April 11, 1950 D, w, ATH 2,503,812

LIMIT SWITCH Filed Aug. 31, 1945 Patented Apr. 11, 1950 r r 2,503,812

UNITED STATES PATENT ornca LIMIT SWITCH Douglas W. Fath, Milwaukee,Wis., assignor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation ofDelaware Application August 31, 1945, Serial No. 613,925 8 Claims. (01.318-466) The invention relates to asystem for controlling added theinertia of the objects carried by the a periodic supply of energy to atranslating debelt, the latter varying depending upon their vice andwhile not limited thereto is particularly weight. The motor may beprovided with a suitadapted to the control of the movement of travelableelectromagnetic brake (not shown). In oring devices which must bearrested in one or more der to stop the belt accurately at the point Dit given positions. is necessary to deenergize the motor and apply theAn object of the invention is to provide means brake at a point A inadvance of the stopping for interrupting current supply to a translatingpoint. The distance between the points A and D circuit in response tomovement into a given poshould be as short as possible so that theslowsition of an element actuated as a function of 10 down period duringwhich the belt is decelerated energization of said translating circuit.is not too much of the total operating time be- Another object is toprovide a system of the tween deliveries at the point D, while the beltapcharacter aforementioned wherein upon interru proaches the point A atfull speed. tion of energy supply to a circuit such circuit can-Conventional limit switches for actuating the not be reenergized duringa predetermined temcontrol of the motor requiremechanical contact poraryperiod. between parts at high relative speeds and are Another object isto provide a system of the therefore subject to impacts Which aredestrucaforesaid character suitable for control of a motive. Inductivedevices used heretofore do not tor-driven traveling device, andparticularly for have this disadvantage but the controlling imstoppageof the device upon its arrival at a eerpulse which they afford is of tooshort duration tain position. for the reliable response ofelectromagnetic cir- Another object is to provide in a conveyingsyscuits controlled by the limit device and in most tem means foreffecting commutation of an eleccases the magnitude of the impulsevaries greatly tric circuit for a. temporary period in response to withspeed. The present invention overcomes the position of an element ofsaid system. these disadvantages as will be apparent from the Otherobjects and advantages of the invention following description of itsoperation. The syswill hereinafter appear. tem comprises an electrontube I1, having a cath- The accompanying drawing is illustrative of anode l1, an anode I1, and a control electrode l1. embodiment of theinvention. The characteristics of the tube are such that it is Figure 1illustrates diagrammatically a system conducting when the potential ofthe control in whichaconveyor is started manually and thereelectrodewith respect to the cathode is zero and after stopped for temporaryperiods after having that it is non-conducting when the potential oftraveled to certain positions. 1 the control electrode becomes negative.Connect- Figs. 2 and 8 are modifications of certain eleed between thecathode I! and the control elecments of the system illustrated in Fig.1; trode PM is a condenser l8. Connected across the Referring to Fig. 1of the accompanying drawbus bars L L is the primary winding H! of a ingthe same illustrate diagrammatically a motor.- potential transformer I9,which has a secondary driven conveyor system which is to be controlledwinding 19 and the primary winding 20 of a so as to stop the conveyor incertain definit potransformer type limit device 20. The transsitions.The conveyor comprise an en less belt former 20 has a magnetic circuitcomprising two ID of suitable construction. Only that much of U-shapedcores 20 and 20 which are mounted the belt and its cooperating mechanismis shown so that the ends of the respective legs face each as inecessary fo an understanding f the upother with an air gap interposedtherebetwcen.

eration of the apparatus. The belt passes over The primary winding 20 ismounted on one le asuitable number of pulleys, two of which (H and ofthe core 20, while a secondary winding 20 I!) are shown. The pulley I2is coupled to a is mounted coaxial with the winding 20 on the drivingmotor I3. At some point (not shown) corresponding leg of the core 20*.Shunting cores objects to be transported, such a boxes [4 and M) aremounted on the conveyor adjacent to the IS, are placed on the belt atregular spaced points. points at which the loads l4 and I5 are placed onThese objects are to be delivered to a point D, the belt I 0,-s0 as tomove between the opposing which is located adjacent to a receivingplatform poles of the cores 20 and 20* thereb shunt ng l8 to which theobjects are to be transferred from at least part of the magnetic flux inthe core i0 the belt. To transfer the objects, the belt must around thecore 20 and from winding 20 thus be stopped. The belt and the connectedmareducing the mutual inductance between the chinery have considerableinertia to which is windings 20 and 20 and reducing the voltageinducedin the winding 20 substantially to zero. The secondary windings l9 and20 of transformers i9 and 20, respectively, are so proportioned and areconnected in series so' that the voltage induced in the winding ||l isequal but opposed to the voltage induced in the winding 20. The seriesconnected two windings are connected in series with a half-waverectifier 2| across the terminals of the condenser l8, with therectifier nearest to that terminal of the condenser l8, which isconnected to the control electrode |1. The winding 20 is shunted by aresistor 22 and the rectifier 2| may also be shunted by a,dischargeresistor 23 of Very high resistance value The tube I1 issupplied with energy from the secondary winding 24 of a transformer 24,whose primary winding 24"- is connected across the bus bars L L Oneterminal of the secondary winding 24 is connected to the cathode l1,

while the other terminal of winding 24 is connected to the anode H inseries with the energizing winding 25 of an electromagnetic relay 25.Connected in parallel with the winding 25 is a smoothing condenser 26.The normally disengaged contacts 25 of relay 25 form a part of thecontrol of the energizing winding 21 of an electromagnetic relay 21,which has normally open contacts 21 and 21. A normally open push buttonswitch 28 is interposed between the line L and one terminal of thecontacts 25*, while the second terminal of the contacts 25 is connectedto one terminal of the winding 21, the other terminal of which isconnected to the line L. A circuit also extends from the connectionbetween the contacts 25 and the push button switch 28, through thecontacts 21 to bus bar L, so that contacts 21 parallel the contacts ofthe push button switch 28. Motor Hi to be controlled by the systemdescribed is shown as adapted to be connected across bus bars L L by thecontacts 21.

The system operates in the following manner. With the bus bars L 1?energized, the primary windings l9, 20 and 24 are also energized. Thisinduces voltages in the secondary windings I9", 20 and 24 Inasmuch asthe sum of the voltages l9 and 2|] is normally zero, no.' voltage isimpressed on the condenser l8 and the potential of the grid I1 is thatof the cathode l1. The tube I1 is therefore conducting, so that acurrent flows through the winding 24 which energizes coil to zero. Thusthe coil l9 impressesa potential upon the condenser l8 and controlelectrode l1 which is negative with respect to the cathode l1 during thehalf-cycles when the winding 24" impresses a potential on the anode |1which is positive with respect to the cathode I1 This negative potentialon the control electrode l'l renders the tube |1 non-conducting andcharges the condenser l8 substantially instantly, the rate of chargebeing only limited by the relatively low forward resistance of rectifier2|. The condenser IB is thus charged to a given potential which isindependent of the speed of the conveyor. The potential of the condenserI8 maintains the tube'l1 non-conducting after the aforementioned core2.0 has traveled beyond the air the winding 25 and closes the contacts25. The

condenser 26 smoothes out the ripples of halfwave rectified energizingcurrent of the winding 25'.

If it is now desired to start the conveyor the push button switch 28 isoperated. This establishes a circuit from line L through contacts ofpush button 28, through contacts 25' and the winding 21, to line L andthe relay 21 is energized, thereby closing the contacts 21, whichcompletes a circuit from line L through contact 21, the motor l3, toline L, and the motor begins to rotate moving the conveyor I0 and theobjects I4 and I5 forward. The contacts 21 also close thereby completinga maintaining circuit from bus bar L through contacts 21 and 25",through the energizing winding 21 to bus bar L thereby paralleling thecontacts 28. The push button switch 28 may therefore be released and therelay 21 will remain energized and the motor will continue to operate.,As soon as the conveyor has moved to a certain position, one of thecores 20 moves into the air gap between the cores 20' and 20", therebyreducing the voltage induced in the gap. The energizing coil 25 isdeenergized to open the contacts 25 and stop the motor Hi. The cycle ofoperation may be repeated by again actuating the push button switch 28.The blocking of the tube I1 is maintained and prevents reenergization ofrelay 25 and of the motor, even though the push button 28 is helddepressed, until the condenser I8 is again discharged. The condenser |8insures a sufficiently long period of deenergization of relay 25 toinsure stoppage of the motor, even though the speed of passage of thecore 20 through the transformer slot is so rapid that the impulsevoltage in the winding 20 is of a duration shorter than the responsetime of relays 25 and 21.

To permit the condenser |8 to discharge during the subsequent period,that is, when the relay 25 is deenergized, the aforementioned highresistance 23 may be connected across the terminals of the rectifier 2|whichpermits discharge current to flow through said resistor, theresistor 22 and the winding 20 in parallel, and the winding l9. In somecases the discharge resistor 23 is not necessary as the slightconduction for reverse current of the rectifier 2| may be sufiicient todischarge the condenser I8, particularly when the intervals .betweensuccessive periods of energization of the relay 25 are relatively long.

The resistor 22 provides a connection between the winding I9 and therectifier 2|. In case of an open circuit or short circuit in thewindings 2|) or 2|! or in the wires leading to the limit device 20, thenegative potential of winding l9 will be impressed upon the controlelectrode |1 by passing through rectifier 2|. This will render the tube|1 non-conducting and will deenergize relay coil 25*, which will opencontacts 25 and stop the motor 3.

In the modification shown in Fig. 2, the transformer 20 of Fig. 1 andits windings are replaced by a transformer 30. The transformer 30 has amagnetic core comprising three parallel legs 30, 30 and 30. An air gap30 is interposed in the third leg 30. The respective ends of the threelegs are connected by yokes in such a mar.- ner that the legs 30* and350' form with the respective yokes a. closed iron circuit, while thelegs 30 and 30 form with the respective yoke a magnetic circuit havingthe air gap 3|] disposed therein. A primary winding 30 is mounted on thecenter leg 30 and connected across the bus bars L and L. A secondarywinding 3|) is mounted on the leg 30 and a, secondary winding 3|]comprising two parts connected in series and located at opposite ends ofthe air gap 30 is mounted on the leg 30. A magnetic core 3|] or aplurality of such magnetic cores are mounted on the machine to take theplace of the magnetic cores 20 in Fig. 1. The windings 30 and 30 areconnected in series with each other across the resistor 22 in such amanner that when the core 30' is interposed in the air gap 30 theresultant of the voltage induced in these windings is of such magnitudeand direction as to reduce the effective voltage on the controlelectrode l1 to a value which renders the tube ll conducting. However,when the core 30* enters the air gap 30, the voltage induced in the coil30 is increased, while the voltage in the coil 30 is decreased as thelarger part of the flux induced in the primary core 30 now passesthrough the leg 30, while the flux which passes through the leg 30 isreduced so that the resultant of the voltage of the windings 3i! and 30'is of such magnitude and direction with respect to the voltage of thewinding I! as to reduce the potential on the control electrode I!sufiiciently to render the tube l'l non-conducting. Hence the cycle ofoperation of the system in accordance with the modification of Fig. 2 isthe same as that aforedescribed in connection with Fig. 1.

Fig. 3 shows still another modification of the system. In this case thetransformer 40 is substituted for the transformer 20 in Fig. 1. Thetransformer 40 comprises a rectangular magnetic circuit 40 having an airgap 40 A primary winding 40 is mounted on one leg of the core.

A secondary winding 40 is mounted on the other parallel leg of the core.The air gap may be substantially short circuited by movable cores 40,which take theplace of cores 20 in Fig, 1. The primary winding 40 isconnected in series with the primary winding ll of a transformer Hacross the lines L L The transformer 4i also has a secondary winding Alwhich is connected in series with the winding 40 across the resistor 22.The voltages normall induced in the windings 41 and 40 are of suchpolarity that their resultant is in opposition to the voltage of thewinding I9 so that the potential impressed upon the control electrode l1renders the tube l1 conducting. However, when the air gap l is reducedupon the core 40= entering it, the voltage induced in the winding 40 isincreased. This also causes an increase of the voltage drop in thewinding 40 with a resulting decrease of the voltage in the primary.winding 4|. This decrease of the primary voltage in winding 4|decreases the voltage in the secondary winding li The result of thedecrease of the voltage in winding 41* and the increase of the voltagein winding 40 is an increase of the total voltage opposing the voltageof the winding IS the increase being of such magnitude as to in turnvary the potential of the control electrode i1 to a value which rendersthe tube l1 non-conducting and to effect operation of the system asdescribed in connection with Fig. 1.

While the operation of the system has been described in connection witha belt or similar conveyor, the same is obviously applicable to otheruses. The device to be operated on might be, for instance, a metalbillet which is moved by a conveyor and which is to be cut to a certainlength by conventional shears. In this cas'e the billet itself couldfunction as the core 20 of Fig. 1, or 30 or 4i! in Figs. 2 and 3,respectively, so that upon its entering the respective air gap thecontroller would actuate a translating device such as a motor orelectromagnet which causes the shear to function to cut the billet tothe desired length, as determined by the position of the transformerrelative to the path of the billet.

The system shown in Fig. 1 may also be used for operation with billets,tubes, or rods, which are 01' non-magnetic material. In that case theeddy currents induced in the billet or the like by the primary winding20, deflect the primary flux from the secondary core of the transformer20. Similar modifications are possible in connection with thetransformers 30 and III of Figs. 2 and 3, respectively, except that inthe latter cases the direction of winding the various secondary coilsaffected would have to be modified to produce the desired polarity ofthe respective voltages.

Instead of starting and stopping the motor l3 by the action of theelectromagnetic switch 21 in Fig. 1, it would also be possible to use acontinuously running motor which cooperates with the traveling devicethrough an electromagnetic gl'utch controlled by the electromagneticswitch cazions will be apparent to those skilled in the ar a I claim:

1. The combination with an electromagnetic device and a control elementto direct changes in the electromagnet condition of said device, ofmeans comprising an electron tube, responsive to said element to effecta given change in condition of said device and remaining effective for agiven minimum period despite movement of said element for directing afurther change in condition of said device, said tube having in acontrolling circuit for said device an anode and a cathode and alsohaving a control electrode, and said means also comprising a capacitorconnected to said electrode and said cathode and having a leakage pathand a variable supply of electrical energy for said capacitor under thecontrol of said control element which element in directing said givenchange in condition of said device effects charging of said capacitorand which when moved to direct a further change permits discharge ofsaid capacitor through said leakage pa h.

2. The combination with an electromagnetic device and a control elementto direct demagnetization of said device, of means comprising anelectron tube responsive to said element to effect the directeddemagnetization of said device and remaining effective for a givenminimum period despite movement of said element for directingmagnetization of said device, said tube having an anode and a cathode inan energizing circuit for said device and also having a controlelectrode, and said means also comprising a capacitor connected to saidcontrol electrode and said cathode and having a leakage path and avariable supply of electrical energy for said capacitor under thecontrol of said control element, which element in directingdemagnetization of said device eifects charging of said capacitor thusto render said tube non-conducting and which when operated to directmagnetization of said device permits discharge of said capacitor throughsaid leakage path thus to render said tube again conducting subject to agiven delay.

3. The combination with an electromagnetic device and a control elementto direct changes in the electromagnetic condition of said device, ofmeans comprising an electron tube, responsive to said element to effecta given change in condition of said device and remaining effective for agiven minimum period despite movement of said element for directing afurther change in condition of said device, said tube having in acontrolling circuit for said device an anode and a cathode and alsohaving a control electrode, and said means also comprising a capacitorconnected to said electrode and said cathode and having a leakage pathand a source of energy for said capacitor variable by said controlelement which element in directing said given change in condition ofsaid electromagnetic device effects charging of said capacitor fornon-conducting conditioning of said tube and which when moved to directa further change permits said capacitor to discharge through saidleakage path, said control electrode having a connection with saidsource of energy independent of said control element to subject saidelectrode to a potential for non-conducting conditioning of said tubebut only when said control element due to circuit disorder loses controlof said source.

4. The combination with an electromagnetic device and a control elementto direct changes in the electromagnetic condition of said device, ofmeans comprising an electron tube, responsive to said element to effecta given change in condition of said device and remaining efiective for agiven minimum period despite movement of said element for directing afurther change in condition of said device, said tube having in acontrolling circuit for said device an anode and a cathode and alsohaving a control electrode, and said means also comprising a capacitorconnected to said electrode and said cathode and having a leakage path,a source of energy for said capacitor, means to vary the energy suppliedto said capacitor from said source which means comprises mutuallyinductive transformer windings, one having connections with saidcapacitor, another having given excitation and the mutual inductancethereof being controllable by said control element which when directingsaid given condition of said electromagnetic device effectin charge ofsaid capacitor for given condition of said tube and which when directinga further change permits discharge of said capacitor for changing theconditioning of said tube subject to a, given delay.

5. The combination with an electromagnetic device and a control elementto direct changes in the electromagnetic condition of said device, ofmeans comprising an electron tube, responsive to said element to eifecta given change in condition of said device and remaining effective for agiven minimum period despite movement of said element for directing afurther change in condition of said device, said tube having in acontrolling circuit for said device an anode and a cathode and alsohaving a control electrode, and said means also comprising a capacitorconnected to said electrode and said cathode, an alternating currentsource, a rectifying circuit for said capacitor extending across saidsource and including a resistor, and mutually inductive windings whosemutual inductance is variable by said control element, one of saidwindings being connected across said resistor and another beingsubjected to given excitation.

6. The combination with an electric motor, electromagnetic control meansfor said motor and a control element for said electromagnetic meansdriven by said motor and directing changes in the electromagneticcondition of said means for different controlling effects on said motor,said element according to its speed being apt upon directing one changeto direct a further change sooner than said change is desired, of meanscomprising an electron tube to control said changes under direction ofsaid element but insuring a given minimum duration of a given changedespite movement of said element to direct a further change, said tubehaving in a controlling circuit for said electromagnetic means an anodeand a cathode and also having a control electrode, and the secondmentioned means comprising in addition to said tube a capacitorconnected vbetween said control electrode and said cathode provided witha leakage path, and a variable supply of electrical energy for saidcapacitor under the control of said motor driven element which elementin directing said given change in condition of said electromagneticmeans effects charging of said capacitor and which when moved to directa further change permits discharge of said capacitor through saidleakage path.

7. The combination with an electromagnetic device and a control elementto direct changes in the electromagnetic condition of said device, ofmeans comprising an electron tube, responsive to said element to effecta given change in condition of said device and remaining eifective for agiven minimum period despite movement of said element for directing afurther change in condition of said device, said tube having in acontrolling circuit for said device an anode and a cathode and alsohaving a control electrode, and said means also comprising a capacitorconnected to said electrode and said cathode, an alternating currentsource, a rectifying circuit for said capacitor extending across saidsource and including a resistor, and mutually inductive windings whosemutual inductance is variable by said control element, one of saidwindings being connected across said resistor and another beingsubjected to given. excitation, said control electrode having aconnection with the source of said energy supply independent of saidcontrol element which connection subjects said electrode to a potentialfor non-conducting conditioning of said tube but only when said controlelement due to circuit disorder loses control over said energy supply.

8. The combination with an electromagnetic device and a control elementto direct changes in the electromagnetic condition of said device, ofmeans comprising an electron tube, responsive to said element to effecta given change in condition of said device and remaining effective for agiven minimum period despite movement of said element for directing afurther change in condition of said device, said tube having in acontrolling circuit for said device an anode and a cathode and alsohaving a control electrode, and said means also comprising a capacitorconnected to said electrode and said cathode, an

alternating current source, a rectifying circuit for said capacitorextending across said source and including a resistor, and mutuallyinductive windings whose mutual inductance is variable by said controlelement, one of said windings being connected across said resistor andanother being subjected to given excitation, said given change incondition of said electromagnetic means being deenergization thereof forstopping of said motor and said electromagnetic means having anassociated manual control which requires a given manual setting forstarting and for each restart of said motor.

DOUGLAS W. FATH.

(References on following page) nmnmcns cmm Numb" The followln:references are of record in the me of this pat 255,526 D STATES PATENTS,287,926 UNITE 2,289,322 Number Name Date ,685 Stevens 1935 Name DateBrown l 'eb. 6, 1 940 Gulllksen Oct. 8, 1940 Lassen Sept 9, 1941 ZeplerJune 30, 1942 Collom July 7. 1942

